1. Field of the Invention
The present invention relates to communication processing systems, orthogonal frequency division multiplexing (OFDM) signal transmitting methods, OFDM transmitters, OFDM receivers, and control stations. In particular, the present invention relates to a communication processing system, an OFDM signal transmitting method, an OFDM transmitter, an OFDM receiver, and a control station capable of transmitting and receiving OFDM signals.
2. Description of the Related Art
The description herein assumes the case where OFDM is used as a radio access scheme, and multicast/broadcast communication (hereinafter referred to as “MBS communication”) is performed. According to Japanese Unexamined Patent Application Publication No. 2007-189646, the description herein also assumes the case where MBS communication is performed and to which a group-wise scrambling method (group scrambling method) is applied. In particular, the description herein assumes the case where two or more pilot subcarriers are arranged in one subcarrier group.
Multicast/broadcast services (MBS) have been standardized. According to the IEEE802.16e standard, by using a cellular network, a multicast/broadcast service assigns a common physical resource to all users (i.e., mobile stations MS owned by the respective users) present in a cell area, and delivers high-quality video streaming, news information, or commercial films. In contrast, communication in which a base station assigns an individual physical resource to one mobile station MS is referred to as “unicast communication”. In the case of MBS communication, the same multicast/broadcast data is transmitted from one or more base stations. A group of base stations that perform MBS communication is defined as a “multicast/broadcast service area”. Generally, as compared to a broadcast service station, one base station included in a multicast/broadcast service area covers a smaller area and has a smaller cell size. Therefore, it is possible to provide location-based information services that are available only in a limited area.
It is also possible to vary the multicast/broadcast service area for each multicast/broadcast service. Since this may cause interference between multicast/broadcast services on a boundary between different multicast/broadcast service areas, it is important to suppress the effect of interference between multicast/broadcast services.
A plurality of pilot signals arranged in one subcarrier group are multiplied by the same scrambling code. Then, channel estimation is made by averaging or interpolating only the plurality of pilot signals in the subcarrier group. Therefore, if positions of pilot subcarriers are the same in adjacent multicast/broadcast service areas, interference caused by pilot signals is strengthened. As a result, it is difficult to obtain a correct channel response, and reception performance is degraded.
Specifically, as illustrated in FIG. 1, in subcarrier group #1 in a transmission signal transmitted from base station 1 belonging to multicast/broadcast service area A, two pilot subcarriers are arranged, and pilot channel signals are present as indicated by a1. In subcarrier group #1 in a transmission signal transmitted from base station 2 belonging to multicast/broadcast service area B, two pilot subcarriers are arranged at the same positions as in the case of subcarrier group #1 for base station 1, and pilot channel signals are present as indicated by b1. A channel response from base station 1 to a mobile station MS is denoted by h_a1, and a channel response from base station 2 to the mobile station MS is denoted by h_b1. A channel response estimate desired by the mobile station MS belonging to multicast/broadcast service area A and adjacent to multicast/broadcast service area B is h_a1 when the mobile station MS is assigned to subcarrier group #1. However, due to interference of pilot channel signals, the actual channel response is a1h_a1+b1h_b1+n1/2+n2/2+n3/2+n4/2. Since a1 is known to the mobile station MS, h_a1+b1* (h_b1/a1)+(n1/2+n2/2+n3/2+n4/2)/a1 can be determined as the channel response by dividing a1h_a1+b1h_b1+n1/2+n2/2+n3/2+n4/2 by a1. Here, n*/2 denotes a noise component, which can be ignored if SNR is high enough. If the mobile station MS is located near multicast/broadcast service area B, since signal power of b1 is large, the second term of the above expression is large. As a result, the quality of a channel response is degraded. The same applies to subcarrier groups #2 and #3.